The skin on the finger of a person carries raised friction ridges on it which follow a distinctive pattern which is unique to that person. Other areas of the skin of the body of a human or other mammal, for example a horse, also carry such raised friction ridges, e.g. the palm of the hand, or the toes or heel of the foot. For convenience the term finger print will be used herein to denote generally any friction skin surface area on the skin, and specifically that raised friction ridge pattern on the final segment or tip of a human finger.
The ridge pattern of a finger print can be visually scanned and analyzed by a skilled observer in order to establish whether the finger print corresponds to one left at the scene of a crime, or to one which is submitted to establish the identity of the person. This requires a highly skilled observer and can be cumbersome and it would be desirable for that operation to be carried out using some form of electronic pattern recognition system.
It has been proposed to scan a finger print using a light source so as to plot the position and type of the characteristics contained within the ridge patterns on the fingerprint. In one type of system, the finger is placed upon one face of a transparent material, for example a sheet of glass, and light is shone onto the opposite face of the material so that the light strikes the underside of the first face at substantially the angle of total internal reflection. Where the ridges of the finger print contact the surface of the material, they alter the angle of total internal reflection and part of the incident light passes through that face of the material and part is reflected back as scattered light. By way of contrast, the incident light is totally internally reflected at those areas where the skin does not contact the material, i.e. those areas where the valleys in the finger print pattern are located.
By illuminating and scanning individual points across the surface of the material, it is possible to build up a picture of light and dark points so as to form an image of the finger print. When viewed along the line corresponding to the exit direction for the totally internally reflected light, a dark image is perceived at the point of contact and a light image where there is no contact, giving a positive image. When viewed from a position significantly offset from the exit path of the totally internally reflected light, the scattered light from the regions where the ridges contact the surface is perceived, but no light is detected from the regions where there is no contact, so that a negative image is perceived.
The image can be scanned electronically to detect the light and dark areas and to convert those into electrical signals which can be recorded as an electronic image of the fingerprint pattern. Either the positive or the weaker negative image can be used to detect the pattern of the ridge lines and features thereon. For convenience, the invention will be described hereinafter in terms of scanning the positive image.
Typical of such systems is that described in U.S. Pat. No. 4,537,484 where the finger is placed upon a channel of transparent material and the light and dark images are scanned to give an electronic record of the image. This is achieved by mounting an elongated light source on a rotating carriage so that an axial strip of the finger print pattern is illuminated and scanned through the under surface of the channel. The carriage also carries a mirror and lens system by which the light reflected from the under surface of the channel can be directed onto an array of light sensitive diodes, arranged axially along the axis of rotation of the carriage, so to generate electronic signals corresponding to the light and dark areas of the image being scanned. The carriage is rotated by a stepper motor to illuminate and scan a series of axial strips of the under surface of the channel so as progressively to build up a total scan of the under surface of the channel. The images perceived by the diode array are read out into a suitable image processor and verified against a record of that persons finger print.
Such a mechanism is complex and expensive to construct and requires the use of high optical quality lenses to focus an elongated image accurately onto the diode array. A further problem is that there may be reflections from the various faces of the channel as well as from the under surface of the face in contact with the finger print. At least part of those other reflections may be internally reflected within the wall of the channel and combine with the totally internally reflected incident light from the desired part of the channel to give a scattered light for scanning by the diode array. This could lead to a loss of definition in the perceived image resulting in rejection of images as unclear. Also, some of the incident light is dissipated by these other reflections reducing the intensity of the light perceived by the diode array.
I have now devised an optical body and scanning device using that body which is reduces the above problems.